Porous metal

ABSTRACT

A porous metal having a much smaller ligament size is provided. The porous metal includes a crystal of an alloy, the alloy containing n or more kinds of elements, n being an integer of 3 or more, the alloy having a composition of each element of from {(100/n)−(75/n)) at % to {(100/n)+(75/n)} at %, and the porous metal having a ligament size of 220 nm or less. The alloy is preferably a high entropy alloy or a medium entropy alloy. In addition, the alloy preferably contains a solid solution of the respective elements.

FIELD OF THE INVENTION

The present invention relates is a porous metal.

DESCRIPTION OF RELATED ART

Conventionally, multiporous porous metals having three-dimensional structures have been developed in order to be used for catalysts, sensors, condensers, and the like, which require large specific surface areas. In particular, in order to make specific surface areas as large as possible, the development of porous metals with small pore and ligament sizes has been advanced by the inventors and others, and porous metals of Fe, FeCo, Ti, V, Nb, Ag, Au, and the like have been obtained (see, for example, Non-Patent Literature 1 to 4).

These porous metals have been produced by a so-called molten metal dealloying method developed by the present inventors (see, for example, Patent Literature 1). This method is a method to obtain a metal member having fine openings by placing a metallic material in a metal bath, the metallic material containing both a second constituent and a third constituent having positive and negative heats of mixing relative to a first constituent, respectively, and including a compound, an alloy, or a non-equilibrium alloy having a melting point that is higher than the solidifying point of the metal bath made of the first constituent, wherein said metal bath is controlled to a temperature lower than a minimum value of a liquidus temperature within a range of compositional variations in which the amount of the third constituent in the metallic material decreases down to a point where the metallic material becomes substantially the second constituent so that the third constituent is selectively dissolved into the metal bath.

Meanwhile, in recent years, a high entropy alloy (High Entropy Alloy; HEA) that is a multicomponent alloy of five or more components, is composed of 5 or more kinds of principal metal elements, and contains respective elements in equal or nearly equal atomic proportions has been proposed (see, for example, Non-Patent Literature 5 or 6). Though the high entropy alloy is a multicomponent alloy the high entropy alloy exists as a solid solution of a single phase or mixed phases thereof, and the configurational entropy is maximized, and, therefore, has high stability, In addition, the high entropy alloy has low Gibbs free energy at high temperatures and also has high thermodynamic stability. By utilizing such features, the application of the high entropy alloy as a material having thermal resistance, high corrosion resistance, high mechanical strength, or high ductility has been progressed (see, for example, Patent Literature 2 or 3).

Further, conventionally, it has been confirmed that when a twin crystal exists on the surface of a crystal, surface diffusion is trapped by the twin interfaces, which causes a delay in surface diffusion (see, for example, Non-Patent Literature 7).

CITATION LIST Non-Patent Literature

-   Non-Patent Literature 1: M. Tsuda et al., “Kinetics of formation and     coarsening of nanoporous a-titanium dealloyed with Mg melt”, J,     Appl. Phys., 2013, 114, 113503 -   Non-Patent Literature 2: J. W. Kim et al., “Optimizing niobium     dealloying with metallic melt to fabricate porous structure for     electrolytic capacitors”, Acta Materialia, 2015, 84, p. 497-505 -   Non-Patent Literature 3: M. S. Kim et al., “Fabrication of     nanoporous silver and microstructural change during dealloying of     melt-spun Al-20 at. %Ag in hydrochloric acid”, J. Mater. Sri., 2013,     48, 5645 -   Non-Patent Literature 4: Y. K. Chen-Wiegart et al., “Structural     evolution of nanoporous gold during thermal coarsening”, Acta     Materialia, 2012, 60, p. 4972-4981 -   Non-Patent Literature 5: B. Cantor et al., “Microstructural     development in equiatomic multicomponent alloys”, Mat. Sci. Eng.,     2004, A 375, 213 -   Non-Patent Literature 6: J. W. Yeh et al., “Nanostructured     high-entropy alloys with multiple principal elements: novel alloy     design concepts and outcomes”, Adv. Eng. Mater., 2004, 6, 299 -   Non-Patent Literature 7: T. Fujita et al., “Atomic observation of     catalysis-induced nanopore coarsening of nanoporous gold”, Nano     Lett, 2014, 14, 3, p. 1172-1177

Patent Literature

-   Patent Literature 1: WO 2011/092909 -   Patent Literature 2:.1P-A-2018-70949 -   Patent Literature 3: WO 2017/098848

SUMMARY OF THE INVENTION

The porous metal disclosed in Non-Patent Literature 1 to 4 has a ligament size of several micrometers or less, a large specific surface area, and excellent characteristics. However, in order to obtain a porous metal having further excellent characteristics, the development of a porous metal having a smaller ligament size and a large specific surface area has been required.

The present invention has been made in view of such a problem, and an object of the present invention is to provide a porous metal having a much smaller ligament size.

The present inventors have confirmed that, when the porous metal disclosed in Non-Patent Literature 1 to 4 is produced, surface diffusion proceeds during the progression of dealloying in order to decrease a surface energy, which makes the ligament size gradually larger. Then, in order to achieve the above-described object, the present inventors have focused on a high entropy alloy as a material of which surface diffusion hardly proceeds during production and which is capable of suppressing the growth of ligaments, and thus completed the present invention.

That is, the porous metal according to the present invention comprises a crystal of an alloy, the alloy containing n or more kinds of elements, n being an integer of 3 or more, the alloy having a composition of each element of from {(100/n)−(75/n)} at % to ((100/n) +(75/n)} at %, and the porous metal having a ligament size of 220 nm or less.

With regard to the porous metal according to the present invention, the alloy is composed of a so-called high. entropy alloy or a medium entropy alloys and the configurational entropy of each element is maximized, and, therefore, the diffusion rate of each element becomes slow, surface diffusion during the production is suppressed, and the growth of ligaments can be suppressed. Accordingly, the porous metal according to the present invention has a smaller ligament size compared with conventional low entropy alloys, not only when the porous metal according to the present invention is a high entropy alloy composed of 5 or more kinds of elements, but also when the porous metal according to the present invention is a medium entropy alloy composed of a smaller number of constituent elements than the high entropy alloy.

The thermodynamic mixing entropy ΔS_(mix) can be provided by the formula (1), and the high entropy alloy can be defined by using the mixing entropy ΔS_(mix) according to the formula (2).

$\begin{matrix} \left\lbrack {{Numerical}\mspace{14mu} {Formula}\mspace{14mu} 1} \right\rbrack & \; \\ {{\Delta \; S_{m\; i\; x}} = {{- R}{\sum\limits_{i = 1}^{n}{x_{i}\ln \; x_{i}}}}} & (1) \\ {{\Delta \; S_{m\; i\; x}} \geq {1.5\; R\mspace{14mu} \left( {{High}\mspace{14mu} {Entropy}\mspace{14mu} {Alloy}} \right)}} & (2) \end{matrix}$

Here, R is the gas constant (8.314 J/K/mol), x_(i) is a molar fraction of the component i, and n is the number of constituent elements.

By using the thermodynamic mixing entropy ΔS_(mix), the medium entropy alloy (Medium Entropy Alloy; MEA) and the low entropy alloy (Low Entropy Alloy; LEA) can be defined according to the formula (3) and the formula (4), respectively.

[Numerical Formula 2]

1.0R≤ΔS _(mix)≤1.5R (MEA)   (3)

ΔS _(mix)≤1.0R (LEA)   (4)

With regard to an alloy with an equiatomic composition composed of 4 elements, ΔS_(mix) is 1.398, R and with regard to an alloy with an equiatomic composition composed of 5 elements, ΔS_(mix) is 1.61 R.

It is preferable that the porous metal according to the present invention has a ligament size of 5 nm or more and/or 100 nm or less. The ligament size refers to a diameter of a cross section perpendicular to the extension direction of the ligament.

It is preferable that the porous metal according to the present invention has a low-energy interface on the surface of the alloy. The low-energy interface is, for example, a coincidence site lattice grain boundary (Coincidence Site Lattice (CSL) boundary) having a low grain boundary energy represented by a twin interface. In this case, surface diffusion becomes slow due to the low-energy interface, and, therefore, the growth of ligaments is further suppressed, which can make the ligament size much smaller.

With regard to the porous metal according to the present invention, it is preferable that the alloy contains one or more refractory metal elements. In particular, it is preferable that the alloy contains (100/n) at % or more of the refractory metal element. In these cases, with regard to surface diffusion, an element having a higher melting point makes the activation energy larger, and, therefore, when the alloy contains more refractory metal elements with a high melting point, the ligament size can become smaller more efficiently.

With regard to the porous metal according to the present invention, it is preferable that the alloy contains a solid solution of respective elements. In this case, the alloy may be a solid solution of a single phase, multiple phases of a solid solution, or a composite phase of a solid solution as a main component and an intermetallic compound. In these cases also, the ligament size can be made much smaller. In addition, the alloy may have a body centered cubic lattice structure, a face centered cubic lattice structure, or a hexagonal close packed structure.

With regard to the porous metal according to the present invention, for example, when the alloy contains 5 kinds of elements, it is preferable that a composition of each element is from 5 at % to 35 at %. In this case, respective elements of the alloy may be composed of, for example, Ti, V, Nb, Mo, and Ta. In addition to the above, for example, the porous metal according to the present invention may be composed of an alloy such as NbTaTiZr, MoTiVZr, HfNbTaTiZr, MoNbTaW, MoNbTaVW, MoNbTaW, MoNbTaVW, CoCrMoNbTi, CrMoNbTaVW, CrMoNbReTaVW, CrMoNbTaTiVWZr, and CrMoNbTaTiVZr.

According to the present invention, it is possible to provide a porous metal having a much smaller ligament size.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are schematic side views illustrating a production method of a porous metal of an embodiment of the present invention.

FIG. 2A is an XRD spectrum of a precursor of a porous metal of an embodiment of the present invention. FIG. 2B is a scanning electron microscope (SEM) photograph near a boundary of a reacted dealloyed region (Dealloyed region) and a region remained as an unreacted precursor (Precursor) on the production of a porous metal of an embodiment of the present invention.

FIGS. 3A to 3D include, with regard to the porous metals of an embodiment of the present invention, SEM photographs of porous metals produced by being immersed for 10 minutes in: (FIG. 3A) a metal bath at 600° C.; (FIG. 3B) a metal bath at 700° C.; (FIG. 3C) a metal bath at 800° C.; and (FIG. 3D) a metal bath at 900° C.

FIGS. 4A and 4B include, with regard to the porous metals of an embodiment of the present invention, XRD spectra of porous metals produced from an FCC precursor alloy: (FIG. 4A) by being immersed for 120 minutes in a metal bath at 800° C.; and (FIG. 4B) by being immersed for 120 minutes in a metal bath at 900° C.

FIGS. 5A and 5B include, with regard to the porous metals of an embodiment of the present invention, TEM photographs and selected area electron diffraction (SAED) patterns (inserted figures) in the cases of: (FIG. 5A) being immersed in a metal bath at 600° C. for 10 minutes; and (FIG. 5B) being immersed in a metal bath at 800° C. for 10 minutes.

FIGS. 6A to 6I include, with regard to the porous metals of an embodiment of the present invention, SEM photographs in the cases of: (FIG. 6A) being immersed in a metal bath at 800° C. for 10 minutes; (FIG. 6B) being immersed in a metal bath at 800° C. for 30 minutes; (FIG. 6C) being immersed in a metal bath at 800° C. for 60 minutes; (FIG. 6D) being immersed in a metal bath at 850° C. for 10 minutes; (FIG. 6E) being immersed in a metal bath at 850° C. for 30 minutes; (FIG. 6F) being immersed in a metal bath at 850° C. for 60 minutes; (FIG. 6G) being immersed in a metal bath at 900° C. for 10 minutes; (FIG. 6H) being immersed in a metal bath at 900° C. for 30 minutes; and (FIG. 6I) being immersed in a metal bath at 900° C. for 60 minutes.

FIG. 7 is a graph showing the relationship between the mean ligament size and the immersion time in a metal bath of each of the porous metals shown in FIGS. 6A to 6I.

FIG. 8 is a graph showing the relationship between the mean ligament size and the normalized value (T_(melting point)/T_(dealloying temperature)) obtained by dividing the melting point of the porous metal by the temperature of the metal bath at the time of dealloying with regard to each of the porous metals of an embodiment of the present invention (HEA) and conventionally obtained porous metals (Nb, FeCr, Ti, FeCo, Fe, V, and Ta).

FIGS. 9A and 9B include, with regard to a porous metal of an embodiment of the present invention, (FIG. 9A) a crystal orientation map and (FIG. 9B) a grain boundary map of a porous metal produced by being immersed in a metal bath at 900° C. for 120 minutes.

FIG. 10 is a graph showing the yield strength of each ligament size of each porous metal of an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the present invention is described hereinafter with reference to an Example and the like.

A porous metal of an embodiment of the present invention comprises a crystal of an alloy, the alloy containing n or more kinds of elements, n being an integer of 3 or more, and having a composition of each element of from {(100/n)−(75/n)} at % to {(100/n)+(75/n)} at %. In addition, a porous metal of an embodiment of the present invention has a ligament size of 220 nm or less.

When the porous metal of an embodiment of the present invention contains, for example, 3 kinds of elements, the composition of each element is from 8.3 at % to 58.3 at %. When the porous metal of an embodiment of the present invention contains 4 kinds of elements, the composition of each element is from 6.2 at % to 43.8 at %. When the porous metal of an embodiment of the present invention contains 5 kinds of elements, the composition of each element is from 5 at % to 35 at %. When the porous metal of an embodiment of the present invention contains 6 kinds of elements, the composition of each element is from 4,1 at % to 29.2 at %. When the porous metal of an embodiment of the present invention contains 7 kinds of elements, the composition of each element is 3.5 at % to 25 at %. Further, it is preferable that the porous metal of an embodiment of the present invention is an alloy composed of a solid solution of respective elements,

The porous metal of an embodiment of the present invention is composed of a so-called high entropy alloy or a medium entropy alloy, and the configurational entropy of each element is maximized, and, therefore, the diffusion rate of each element becomes slow, surface diffusion during the production is suppressed, and the growth of ligaments can be suppressed. Accordingly, the porous metal of an embodiment of the present invention has a smaller ligament size as compared with conventional low entropy alloys.

Hereinafter, as an Example, porous metals of an embodiment of the present invention were produced, and observation and measurement of ligament sizes and the like were performed.

EXAMPLE 1

Porous metals of an embodiment of the present invention were produced by using the molten metal dealloying method described in. Patent Literature 1. First, as a precursor alloy, a precursor alloy containing 20 at % in total of the 5 elements of Ti, V, Nb, Mo, and Ta along with 80 at % of Ni was prepared. The precursor alloy contained Ti, V, Nb, Mo, and Ta in approximately equal atomic proportions. The precursor alloy was melted by arc melting such that each component was mixed, and from the alloy after solidification, a sheet-like precursor with a thickness of 0.5 mm was cut out. In addition, a metal bath composed of Mg was prepared. In the Example, the precursor alloy had a fraction of Ni of 80 at %, but in order to produce porous metals, a fraction of Ni may generally be from 30 to 80 at %. When the fraction of Ni is 80 at %, the smallest ligaments can be obtained. When a fraction of Ni is 50 at %, a porous structure having a large specific surface area per area can be obtained. When a fraction of Ni is 30 at %, a porous material having the highest mechanical strength can be obtained.

Here, Mg is a first constituent in the molten metal dealloying method, 5 elements of Ti, V, Nb, Mo, and Ta are second constituents, and Ni is a third constituent. The five elements of Ti, V, Nb, Mo, and Ta have a positive heat of mixing relative to Mg and at the same time, they are immiscible with Mg. Ni has a negative heat of mixing relative to Mg. The melting point of Mg used in the metal bath was 650° C., so in order to make dealloying at 600° C. possible when the metal bath at 600° C. was used, an Mg alloy molten metal having a lower melting point that was lowered by adding 10 at % of Ca was used.

Next, a porous metal was produced according to a method shown in FIGS. 1A to 1C by using the prepared. precursor and the metal bath. That is, as shown in FIG. 1A, a precursor 11 is immersed in a metal bath 2 composed of Mg that is melted by being heated with an induction coil 1. Thereby, Ni can be selectively eluted into the metal bath 2, and a porous metal 10 a having fine pores can be obtained. In the porous metal 10 a. removed from the metal bath 2, an Mg phase remains inside the fine pores. Then, as shown in FIG. 113, the porous metal 1.0 a is immersed in an aqueous solution of nitric acid (HNO₃) 3 to remove the Mg phase within the fine pores. Thus, as shown in FIG. 1C, a porous metal 10 composed of 5 elements of Ti, V, Nb, Mo, and Ta can be obtained.

Porous metals were produced by setting a temperature of the metal bath to a range of 600° C. to 900° C. and setting an immersion time into the metal bath to a range of 10 minutes to 120 minutes. The measurement result by an X-ray diffraction (XRD) method with regard to the precursor used for the production of the porous metals is shown in FIG. 2A. In addition, the observation result by a scanning electron microscope (SEM) with regard to a region near a boundary of a reacted dealloyed region (Dealloyed region) immersed in a metal bath at 850° C. for 10 minutes and a region remained as an unreacted precursor (Precursor) is shown in FIG. 2B.

As shown in FIGS. 2A and 2B, with regard to the precursor, a single phase of a solid solution having a face centered cubic lattice structure (FCC) was observed, and the lattice constant a of the FCC was 3.6070. The blended amount of each element is shown in Table 1. As shown in Table 1, each phase of the FCC contains the 5 elements of Ti, V, Nb, Mo, and Ta in approximately equal atomic proportions.

TABLE 1 Ti V Nb Mo Ta Ni Precursor 2.74 4.34 4.97 5.71 2.24 80

The observation result by a scanning electron microscope (SEM) of each porous metal produced from the FCC by being immersed in a metal bath at 600, 700, 800, or 900° C. for 10 minutes is shown in FIGS. 3A to 3D. As shown in FIGS. 3A to 3D, under any condition, it can he confirmed that the structure of the produced porous metal is uniform.

The measurement results by an X-ray diffraction (XRD) method with regard to a porous metal produced by being immersed in a metal bath at 800° C. for 120 minutes and a porous metal produced by being immersed in a metal bath at 900° C. for 120 minutes are shown in FIGS. 4A and 4B, respectively. As shown in FIGS. 4A and 4B, each of the produced porous metals is composed of a solid solution phase with a body centered cubic lattice structure (BCC), and Ni is not present; therefore, it is thought that the reaction has been completed. The lattice constant a of the BCC was 3.2300.

With regard to a BCC porous metal obtained by being immersed in a metal bath at 900° C. for 10 minutes, the blended amount of each element is shown in Table 2. As shown in Table 2, the porous metal contains the 5 elements of Ti, V, Nb, Mo, and Ta in approximately equal atomic proportions, and it is thought that they form a high entropy alloy (HEA). Accordingly, it was confirmed that even when a precursor having a complex combination of elements was utilized, only Ni was selectively dissolved. In addition, it is thought that the content of refractory metal elements affects the sizes of ligaments and pores.

TABLE 2 Ti V Nb Mo Ta Ni Precursor 9.01 ± 22.38 ± 26.33 ± 32.17 ± 9.84 ± 0.27 ± 0.05 0.16 0.17 0.12 0.34 0.21

The observation results by a transmission electron microscope (TEM) of a porous metal produced by being immersed in a metal bath at 600° C. for 10 minutes and a porous metal produced by being immersed in a metal bath at 800° C. for 10 minutes are shown in FIGS. 5A and 5B, respectively. Each of the inserted figures in FIGS. 5A and 5B is an SAED pattern corresponding to a body-centered cubic lattice of each of the produced porous metals. As shown in FIGS. 5A and 5B, it can be confirmed that even when the immersion time is short, the BCC porous structure of a nanometer size is formed. In addition, as shown in each SAED pattern, it can also be confirmed that polycrystalline ligaments are formed. With regard to the BCC porous metal obtained by being immersed in a metal bath at 600° C. for 10 minutes, the blended amount of each element is shown in Table 3.

TABLE 3 Ti V Nb Mo Ta Ni Precursor 9.17 ± 17.43 ± 20.67 ± 24.53 ± 9.77 ± 18.42 ± 0.21 0.23 0.31 0.45 0.50 1.23

As shown in FIGS. 5A and 5B, it was confirmed that the obtained porous metals have very small sizes of ligaments and pores. With regard to these microphotographs, analyses were performed by using image processing software “ImageJ.” As a result, the mean ligament size of the porous metal obtained by being immersed in the metal bath at 600° C. for 10 minutes was 10 n m, which was an extremely small value. The mean ligament size of the porous metal obtained by being immersed in the metal bath at 800° C. for 10 minutes was 34 nm, and the mean ligament size of the porous metal obtained by being immersed in the metal bath at 900° C. for 10 minutes was 99 nm. As shown in Table 3, Ni remains in the porous metal obtained by being immersed in the metal bath at 600° C. for 10 minutes, and, therefore, it is thought that the porous metal is still in the middle of the reaction, but the porous metal with a BCC porous structure and very small ligament sizes was obtained. Meanwhile, as shown in Table 3, BCC ligaments contain large amounts of refractory metal elements, i.e., V and Mo. In addition, from Table 3, it can be understood that the BCC phase corresponds to a 6-component high entropy alloy.

The observation results by a scanning electron microscope (SEM) of BCC porous metals obtained by being immersed in the metal bath at 800° C., 850° C., and 900° C. for 10 minutes, 30 minutes, and 60 minutes are shown in FIGS. 6A to 6I, respectively. In addition, each mean ligament size (Ligament size) was obtained by using image processing software “ImageJ” on each microphotograph shown in FIGS. 6A to 6I, and the mean ligament size was plotted against the immersion time in the metal bath. The resulting plot is shown in FIG. 7.

As shown in FIGS. 6A to 6I, it was confirmed that each porous metal had the ligament and pore sizes of several 10 nm to several 100 nm, which were very small. In addition, as shown in FIG. 7, it was confirmed that, when the temperature of the metal bath remained constant, the longer the immersion time became, the larger the ligament size became. However, with regard to the ligament size, when the immersion time was from 10 minutes to 60 minutes, in the case where the temperature of the metal bath was 800° C., the ligament sizes ranged from 34 nm to 70 nm, in the case where the temperature of the metal bath was 850° C., the ligament sizes ranged from 69 nm to 158 nm, and in the case where the temperature of the metal bath was 900° C., the ligament sizes ranged from 99 nm to 212 nm. Accordingly, it was confirmed that even though the heat treatment temperatures (the temperatures of the metal bath) were high, the ligament sizes remained very small values. Each of these porous metals corresponds to a high entropy alloy.

The mean ligament size of each of the porous metals (high entropy alloys; HEAs) obtained by being immersed in the metal bath at 600° C., 800° C., 850° C., and 900° C. for 10 minutes, as well as each of the porous metals obtained by being immersed in the metal bath at 700° C., 800° C., 850° C., and 900° C. for 60 minutes was plotted against the normalized value (T_(melting point)/T_(dealloying temperature)) obtained by dividing the melting point of the porous metal by the temperature of the metal bath at the time of dealloying. The resulting plot graph is shown in. FIG. 8. The melting point of each porous metal (HEA) is a value estimated by the rule of mixtures (rule of mixtures; ROM). In addition, FIG. 8 also plots the ligament sizes of other porous Metals (which are Nb, FeCr, Ti, FeCo, Fe, V, and Ta, and are materials not corresponding to a medium entropy alloy or a high entropy alloy) obtained by the inventors and others thus far by being immersed in the metal bath for 10 minutes and 60 minutes for purposes of comparison (see Non-Patent Literature 1 to 4 and the like).

As shown in FIG. 8, each conventional porous metal is arranged along each of the straight lines shown by a solid line and a broken line in the figure, regardless of the kind of the metals and when the immersion time into the metal bath is constant, and it can be assumed to be consistent with the power law. On the other hand, each porous metal composed of HEA is away from each straight line of each conventional porous metal, and it was confirmed that each porous metal composed of HEA is arranged along a straight line different from the straight lines of conventional porous metals. It was confirmed that the porous metal of HEA had smaller ligament sizes than conventional porous metals when the values of T_(melting point)/T_(dealloying temperature) were the same.

A crystal orientation map and a grain boundary map of a porous metal produced by being immersed in the metal bath at 900″C for 120 minutes are shown in FIGS. 9A and 9B. Each bright line in the grain boundary map of FIG. 9B shows a coincidence site lattice grain boundary (Coincidence Site Lattice (CSL) boundary). As shown in FIGS. 9A and 9B, in the porous metal of an embodiment of the present invention, it was confirmed that at grain boundaries constituting the ligaments, a proportion of CSL grain boundaries with a low grain boundary energy is higher than that of general grain boundaries with a high degree of hardness. The mean proportion of CSL grain boundaries in the porous metal shown in FIGS. 9A and 9B is 0.538. Since surface diffusion becomes slower due to a twin crystal that is one of the CSL grain boundaries (see, Non-Patent Literature 7), in the porous metal of an embodiment of the present invention having a high proportion of CSL grain boundaries, it is thought that the growth of ligaments is further suppressed, which makes the ligament size much smaller.

With regard to porous metals produced by being immersed in the metal bath at 600″C to 900° C. for 10 minutes to 120 minutes, the evaluation tests of the mechanical characteristics of ligaments were performed by a nanoindentation method in accordance with ISO 14577. From the obtained test results, the yield strength of each ligament was obtained, which is shown in FIG. 10. As shown in FIG. 10, the yield strengths of ligaments with sizes of 10 nm to 462 nm ranged from 1.9 GPa to 10.8 GPa, and it was confirmed that the porous metal of an embodiment of the present invention has greater strength than a gold porous body (Nanoporous gold) with the same ligament size.

REFERENCE SIGNS LIST

-   1: Induction coil -   2: Metal bath -   3: Aqueous solution of nitric acid (HNO₃) -   10, 10 a: Porous metal -   11: Precursor 

1. A porous metal comprising a crystal of an alloy, the alloy containing n or more kinds of elements, n being an integer of 3 or more, the alloy having a composition of each element of from {(100/n)−(75/n)} at % to {(100/n)+(75/n)} at %, and the porous metal having a ligament size of 220 nm or less.
 2. The porous metal according to claim 1, wherein the alloy contains one or more refractory metal elements.
 3. The porous metal according to claim 2, wherein the alloy contains (100/n) at % or more of the refractory metal element.
 4. The porous metal according to claim 1, wherein the alloy contains a solid solution of the respective elements.
 5. The porous metal according to claim 1, wherein the alloy contains 5 kinds of elements and the composition of each element is from 5 at % to 35 at %.
 6. The porous metal according to claim 5, wherein the respective elements of the alloy comprise Ti, V, Nb, Mo, and Ta.
 7. The porous metal according to claim 1, wherein the alloy is a high entropy alloy or a medium entropy alloy.
 8. The porous metal according to claim 1, wherein the ligament size is 5 nm or more and/or 100 nm or less. 